By Kim Eun-jung SEOUL, Sept. 21 (Yonhap) -- South Korea's Navy on Friday fired warning shots against North Korean fishing boats operating south of the western sea border, but there was no clash with the North's navy, military officials said.

A total of six North Korean boats had violated the sea border since 11 a.m. Two South Korean patrol vessels were sent to the area before broadcasting radio warnings twice and then firing off dozens of warning shots from around 3 p.m., officials said.

All of the North Korean boats returned to north of the maritime boundary, known as the Northern Limit Line, around 4 p.m. North Korean patrol ships, which were on the north side of the NLL, did not fire back, officials said.

It is the first time the South Korean military has fired off warning shots against North Korean vessels since November 2010, shortly before the North shelled the front-line Yeonpyeong Island in an unprovoked attack.

The latest move comes after the Joint Chiefs of Staff earlier in the day said it has raised the alert status near the western sea border after detecting a growing number of North Korean vessels fishing south of the NLL, the de facto maritime boundary between the two Koreas.

Seven boats crossed the demarcation line twice on Sept. 12 and retreated after warning messages from South Korean patrol boats, and a group of two or three fishing boats repeatedly appeared near the NLL on Sept. 14, 15 and 20, according to the JCS.

"Our military is closely monitoring to figure out the intention of the North Korean boats' border crossings," the official said, asking anonymity due to the sensitivity of the issue. "If North Korean boats repeatedly cross the NLL for fishing, the military will promptly and sternly respond, without hesitation."

The North's military has not yet shown any special signs of provocation, the official added.

About 100 North Korean boats and 300 Chinese ships are currently fishing for crabs in waters north of the NLL in the peak crab season, according to officials.

Another official questioned the North's intention of frequent border crossing at this time with the presidential elections only three months away.

"The North's NLL violations, four times this year alone, clearly seem to have a reason," the official said, adding that the North "may try to disturb South Korea by creating military tension ahead of (December's) presidential election."

The NLL has been the scene of several bloody skirmishes between the navies of the two Koreas. Most recently, tensions escalated after Pyongyang shelled Yeonpyeong Island, killing four South Koreans.

North Korea does not recognize the NLL, arguing it was unilaterally drawn by the U.S.-led United Nations forces at the conclusion of the 1950-53 Korean War that ended in a truce. It has demanded that a new border be drawn further south.